1. Field of the Invention
The present invention relates to a switching power supply device that generates and outputs a predetermined voltage by a switching operation, and is capable of a stable control at a time when a load is light.
2. Description of the Related Art
A switching power supply device that controls an output voltage by performing an ON/OFF control for a switching element has been heretofore used for OA equipment, consumer appliances and the like. In recent years, efficiency enhancement of the switching power supply device has been required from viewpoints of considering the environment and saving energy. A control circuit that controls the switching element in the switching power supply device is usually composed of a one-chip integrated circuit, and includes, in an inside of the integrated circuit, a starting circuit for starting the integrated circuit concerned.
A conventional quasi-resonant switching power supply device shown in FIG. 1 includes: an alternating current power supply 1; abridge rectifier 2; a capacitor 3 for a normal filter; a transformer 4; a switching element 5; a rectifying diode 6; an output capacitor 7, an error amplifier 8; a light emitting diode (LED) 9a and phototransistor 9b of a photocoupler; capacitors 10 and 14; a diode 11; a backup capacitor 12; a resistor 13; a resonance capacitor 15; and a control unit 50 for controlling the switching element 5.
The transformer 4 has a primary winding P, a secondary winding S and an auxiliary winding D, and transmits energy from a primary-side circuit of the switching power supply device to a secondary-side circuit thereof. Moreover, the switching element 5 is connected to the primary winding P of the transformer 4. The auxiliary winding D, the diode 11 and the backup capacitor 12 compose an auxiliary power supply circuit.
Moreover, the switching element 5, the resonance capacitor 15 and the control unit 50 are provided, for example, in a one-chip semiconductor device. Then, the semiconductor device includes: as external terminals, an input terminal of the switching element 5 (Drain terminal); an output terminal of the switching element 5 (Source terminal); an input terminal of the auxiliary power supply circuit (Vcc terminal); a feedback signal input terminal (FB terminal); an overcurrent protection terminal (OCP terminal); a zero current detection terminal (ZCD terminal); and a ground terminal of the control unit 50 (GND terminal). Note that the control unit 50 includes: a StartUp terminal connected to the Drain terminal; the Vcc terminal; the FB terminal; the GND terminal; the OCP terminal; the ZCD terminal; and a DRV terminal for outputting a control signal to the switching element 5.
The error amplifier 8 is connected between a power supply output terminal Vout and a ground terminal Gnd of the secondary-side circuit, and controls a current flowing through the LED 9a of the photocoupler in response to a difference between an output voltage Vout and an internal reference voltage of the error amplifier 8 concerned. A resistor is connected in parallel to the LED 9a of the photocoupler, and the LED 9a gives feedback of an error with respect to the reference voltage of the secondary-side circuit to the primary-side circuit. Moreover, the phototransistor 9b of the photocoupler operates in response to light of the LED 9a of the photocoupler. A collector of the phototransistor 9b of the photocoupler is connected to the FB terminal of the control unit 50, and an emitter thereof is grounded. With this configuration, the phototransistor 9b of the photocoupler outputs a feedback signal to FB terminal of the control unit 50. Therefore, the switching power supply device can supply power corresponding to variations of a load.
As described above, the auxiliary power supply circuit is composed by connecting the diode 11 and the backup capacitor 12 to the auxiliary winding D. Moreover, the auxiliary power supply circuit rectifies and smoothes a voltage induced in the auxiliary winding D, and charges the backup capacitor 12 owned thereby to then supply power to the Vcc terminal of the control unit 50. Moreover, the voltage induced in the auxiliary winding D is inputted to the ZCD terminal of the control unit 50 thorough the resistor 13 without being rectified or smoothed.
A voltage induced in the secondary winding S during an OFF period of the switching element 5 is rectified and smoothed by the rectifying diode 6 and the output capacitor 7, and is outputted as an output voltage of the secondary-side circuit from such a Vout terminal to the load.
Moreover, the primary-side circuit includes an LC resonance circuit composed of inductance of the primary winding P of the transformer 4, and capacitance of the resonance capacitor 15 connected in parallel to the switching element 5. It is also possible to compose the resonance capacitor 15 only of parasitic capacitance of the switching element 5. Although the resonance capacitor 15 is connected in parallel to the switching element 5, the resonance capacitor 15 exerts the same effect even in the case of being attached in parallel to the primary winding P.
The control unit 50 outputs the control signal from the DRV terminal, thereby drives a gate of the switching element 5 to turn on/off the switching element 5, and generates a direct current voltage, which is smoothed on the secondary winding S side of the transformer 4, between the power supply output terminal Vout and the ground terminal Gnd. Specifically, the transformer 4 generates counter electromotive force by a drain current flowing therethrough during an ON period of the switching element 5. As a result, a current flows through the secondary winding S side, and energy is stored in the transformer 4. Thereafter, the switching element 5 is turned off; however, the energy stored in the transformer 5 flows a current to the output capacitor 7 through the rectifying diode 6 on the secondary winding S side of the transformer 4 during an OFF period of the switching element 5. In such a way, the direct current voltage smoothed on the secondary winding S side of the transformer 4 is generated between the power supply output terminal Vout and the ground terminal Gnd.
When discharge of the energy, which is stored in the transformer 4, to such a smoothing circuit on the secondary winding S side is ended, the current flowing through the rectifying diode 6 becomes zero. As a result, a voltage between source and drain terminals of the switching element 5 drops. Then, oscillations start in the LC resonance circuit of the transformer 4. At this time, in the auxiliary winding D of the transformer 4, a voltage corresponding to a drain voltage with an amplitude proportional to the number of turns thereof is generated. The drain voltage of the switching element 5 oscillates while taking, as a center, a direct current voltage generated between terminals of the smoothing capacitor 3 when an alternating current input from the alternating current power supply 1 is rectified. As opposed to this, since the input terminal ZCD for detecting the zero current is connected to the auxiliary winding D of which polarity is reversed from that of the primary winding P of the transformer 4, a zero current detection signal with an oscillation waveform in which 0 V is taken as a center is inputted to the imputer terminal ZCD concerned. In such a way, the control unit 50 outputs a signal of an H level to the gate terminal of the switching element 5, and turns on the switching element 5 again.
Here, in the case where the load (not shown) connected between the Vout terminal and the Gnd terminal is in a light load state where impedance is high, an ON width of the control signal for the switching element 5 is narrowed. Hence, the ON period of the switching element 5 is short, and the maximum value of the drain current thereof is low. Therefore, the energy stored in the transformer 4 is small, and resetting of the transformer 4 is ended in a relatively short period. Hence, a period while a value of the voltage between the drain and source terminals of the switching element 5 is high is shortened, and the current flowing through the rectifying diode 6 connected to the secondary winding S of the transformer 4 becomes zero in a short period.
Meanwhile, in the case where the load connected between the Vout terminal and the Gnd terminal is in a heavy load state where the impedance is low, the ON width of the control signal outputted by the control unit 50 is widened. Hence, the ON period of the switching element 5 is lengthened, and the maximum value of the drain current thereof rises. Therefore, the energy stored in the transformer 4 is increased, and a resetting period of the transformer 4 is lengthened. Hence, the period while the value of the voltage between the drain and source terminals of the switching element 5 is high is lengthened, and the current flowing through the rectifying diode 6 connected to the secondary winding S of the transformer 4 continues to flow therethrough during a relatively long period.
Moreover, as shown in FIG. 2, the control unit 50 of the conventional quasi-resonant switching power supply device includes, in an inside thereof, an internal power supply 51, an inverter 52, a hysteresis comparator 54, a flip-flop 56, a starting circuit 57, a constant current source 60, a transistor 61, an FB comparator 62, an OCP comparator 63, an OR gate 64, an AND gate 65, an oscillator 66, a second inverter 67, first and second drive circuits 68 and 69, first and second driving switching elements 70 and 71, a BD comparator 84, a bottom detection unit 85, and a second OR gate 86.
The internal power supply 51 starts the control unit 50 based on power supplied from the Vcc terminal, and supplies, to the entirety of the control unit 50, power necessary for operations thereof. Moreover, the internal power supply 51 detects an output of the hysteresis comparator 54, and operates in the case where the output of the hysteresis comparator 54 is a signal of the high (H) level, but stops operating and stops the supply of the power to the entirety of the control unit 50 in the case where the output is a low (L) level.
The hysteresis comparator 54 outputs the signal of the H level in the case where a voltage of the Vcc terminal is 16.5 V as a starting voltage value or more. Thereafter, when the voltage of the Vcc terminal drops to 10 V as the lowest operation voltage value or less, the hysteresis comparator 54 outputs the signal of the L level.
The inverter 52 inverts the output of the hysteresis comparator 54, and outputs the inverted output to a switch 81 in the starting circuit 57 to be described later.
The starting circuit 57 is composed of a constant current source 80 and a switch 81, and flows therethrough a starting current in order to supply the power to the internal power supply 51. Here, an input terminal of the constant current source 80 is connected to the StartUp terminal, and receives the supply of the power from the external Drain terminal. In the case where the switch 81 is turned on, the starting circuit 57 supplies the current, which is generated by the constant current source 80, through the Vcc terminal to the backup capacitor 12 of the auxiliary power supply circuit 30, and charges the backup capacitor 12. Moreover, the switch 81 in the starting circuit 57 switches on in the case where the output of the inverter 52 is the signal of the H level, and switches off in the case where the output of the inverter 52 is the signal of the L level. Hence, the starting circuit 57 turns on the switch 81 and supplies the starting current to the control unit 50 in the case where the voltage of the Vcc terminal drops to 10 V or less and it is necessary to restart the control unit 50.
The constant current source 60 generates a feedback voltage, which comes from the secondary-side circuit, at the FB terminal by the phototransistor 9b of the photocoupler and the capacitor 10, which are connected to the FB terminal on the outside of the control unit 50.
In the transistor 61, a base thereof is connected to the FB terminal. Then, the transistor 61 turns on in response to the feedback voltage of the FB terminal, and an emitter current flows therethrough.
The OCP terminal is connected to the Source terminal on the outside of the control unit 50. A voltage corresponding to an amount of a current flowing through the switching element 5 is applied to the OCP terminal, and the OCP terminal outputs a voltage signal to the FB comparator 62 and the OCP comparator 63.
The FB comparator 62 outputs an H signal in the case where the voltage signal outputted from the OCP terminal exceeds a voltage signal corresponding to the amount of the current flowing through the transistor 61. In such a way, when a voltage value of the voltage signal outputted form the OCP terminal exceeds a voltage value corresponding to a feedback amount from the secondary-side circuit, which is shown on the FB terminal, the FB comparator 62 outputs the signal of the H level to an R terminal of the flip-flop 56 through the OR gate 64. As a result, the switching element 5 is turned off, and an output voltage value of the secondary-side circuit is constantly controlled.
In the case where the voltage signal inputted to the OCP terminal exceeds a predetermined voltage value, the OCP comparator 63 determines that the amount of the current flowing through the switching element 5 is an overcurrent, and outputs an H signal. Then, this signal of the H level is inputted through the OR gate 64 to the R terminal of the flip-flop 56.
In the case where at least one of the FB comparator 62 and the OCP comparator 63 outputs an H signal to the OR gate 64, the OR gate 64 outputs an H signal to the R terminal of the flip-flop 56.
The oscillator 66 generates a maximum duty cycle signal that decides a maximum duty cycle of the switching element 5, and then outputs the maximum duty cycle signal to the AND gate 65. Moreover, the oscillator 66 generates a clock signal that decides an oscillation frequency of the switching element 5. This clock signal is outputted to an S terminal of the flip-flop circuit 56 through the second OR gate 86. In such a way, the oscillator 66 restricts the ON width of the switching element 5 when the load is excessive, and thereby can prevent the overcurrent from flowing therethrough.
The flip-flop 56 outputs a control signal from an output terminal (Q terminal) thereof based on the clock signal inputted to the S terminal and on the signal inputted to the R terminal. The Q terminal of the flip-flop 56 is connected to an input terminal of the AND gate 65. Moreover, an output terminal of the AND gate 65 is connected to the first and second drive circuits 68 and 69 through the second inverter 67. The first drive circuit 68 is connected to a gate terminal of the first driving switching element 70 made of a P-type MOSFET. Moreover, the second drive circuit 69 is connected to a gate terminal of the second driving switching element 71 made of an N-type MOSFET. The first and second driving switching elements 70 and 71 are driven alternately in response to an output of the AND gate 65, whereby the switching element 5 is controlled to be turned on/off.
As mentioned above, the BD comparator 84 compares, with a predetermined value, a voltage value of the zero current detection signal with the oscillation waveform in which 0 V is taken as the center, and outputs a comparison result to the bottom detection unit 85.
The bottom detection unit 85 performs zero cross detection for the zero current detection signal, which is applied to the input terminal ZCD, based on the output of the BD comparator 84. Then, the bottom detection unit 85 outputs a signal of the H level to the S terminal of the flip-flop 56 through the second OR gate 86 at timing when the drain voltage of the switching element 5 becomes the lowest voltage (bottom). In such a way, a switching operation in a state where the current flowing through the transformer 4 is zero, that is, soft switching can be realized.
In the case where at least one of the oscillator 66 and the bottom detection circuit 85 outputs the H signal, the second OR gate 86 outputs the H signal to the S terminal of the flip-flop 56.
Next, a description will be made of operations of the conventional switching power supply device. First, a sinusoidal voltage outputted by the alternating current power supply 1 is rectified by the bridge rectifier 2, passes through the capacitor 3, and is inputted to the Drain terminal of the switching element 5 through the primary winding P of the transformer 4. Meanwhile, since the switch 81 is turned on, the starting circuit 57 supplies a current of the constant current source 80 to the backup capacitor 12 of the auxiliary power supply circuit and charges the backup capacitor 12 until the voltage of the Vcc terminal exceeds 16.5 V. When the voltage of the Vcc terminal exceeds 16.5 V, and the internal power supply 51 starts to operate and starts to supply the power to the control unit 50, then the starting circuit 57 turns off the switch 81, and stops supplying the starting current.
When the voltage of the Vcc terminal exceeds 16.5 V, and the operations of the control unit 50 are started, then the switching element 5 starts a switching operation. Therefore, the energy starts to be supplied to the respective windings of the transformer 4, and currents flow through the secondary winding S and the auxiliary winding D.
The alternating current flowing through the secondary winding S is rectified and smoothed by a rectifying/smoothing circuit composed of the rectifying diode 6 and the output capacitor 7, and thereby becomes a direct current. Then, this direct current is outputted from the Vout terminal to the external load.
Thereafter, the switching operation of the switching element 5 is repeated, whereby the output voltage of the Vout terminal gradually rises. Then, when the output voltage of the Vout terminal reaches the reference voltage set in the error amplifier 8, the current flowing through the LED 9a of the photocoupler is increased. Then, a current flowing through the phototransistor 9b of the photocoupler is increased. As a result, the capacitor 10 is discharged, and the voltage of the FB terminal drops. In such a way, the control unit 50 controls the switching element 5 to stabilize the output voltage of the Vout terminal. During a period while the switching operation of the switching element 5 is being stopped, a voltage VFB of the FB terminal rises in such a manner that a current generated by the constant current source 60 charges the capacitor 10.
The alternating current flowing through the auxiliary winding D is rectified and smoothed by the diode 11 and the backup capacitor 12, is fully used as an auxiliary power supply of the control unit 50, and supplies the power to the Vcc terminal. As mentioned above, when the Vcc terminal reaches the starting voltage (16.5 V) once, the switch 81 in the starting circuit 57 is turned off. Therefore, the supply of the power to the Vcc terminal after the start of the control unit 50 is performed by the auxiliary power supply circuit. A polarity of the auxiliary winding D is the same as that of the secondary winding S, and accordingly, the voltage of the Vcc terminal becomes proportional to the output voltage of the Vout terminal.
When the load connected to the Vout terminal becomes light, the current flowing through the LED 9a of the photocoupler is increased in response to the error of the Vout voltage with respect to the reference voltage set in the error amplifier 8. Then, the current flowing through the phototransistor 9b of the photocoupler is increased. As a result, the capacitor 10 is discharged, and the voltage of the FB terminal drops. In such away, the flip-flop 56 is reset, and the control unit 50 controls the switching element 5 to shorten an ON time (ON width).
Moreover, as mentioned above, if the ON time of the switching element 5 is controlled to be shortened at the time when the load is light, then the maximum value of the drain current thereof is lowered, and accordingly, the energy stored in the transformer 4 is also decreased. Therefore, the resetting of the transformer 4 is ended in a relatively short period. Hence, the period while the value of the voltage between the drain and source terminals of the switching element 5 is high is shortened, and the current flowing through the rectifying diode 6 connected to the secondary winding S of the transformer 4 becomes zero in a short period. Thereafter, the flip-flop 56 is set by the bottom detection unit 85, and accordingly, an OFF time of the switching element 5 is also shortened in a similar way to the ON time thereof. Therefore, the frequency of the switching element 5 rises.
While the voltage of the FB terminal is dropping and the oscillation of the switching element 5 is being stopped, the current flowing through the LED 9a of the photocoupler is decreased. Then, following such a decrease, the current flowing through the phototransistor 9b of the photocoupler is decreased. In such a way, the capacitor 10 is charged by the constant current source 60, and the voltage of the FB terminal rises. The switching power supply device repeats the above-described operations, and when the load is light, controls the voltage by raising the switching frequency of the switching element 5.
FIG. 3 is a waveform chart of the respective portions of the conventional quasi-resonant switching power supply device when the load is light. As shown in FIG. 3, the control unit 50 outputs the high-frequency control signal from the DRV terminal. Therefore, the switching frequency of the switching element 5 rises, and the maximum value of a current Ids between the drain and source terminals becomes a small value. Moreover, FIG. 4 is a switching operation waveform chart of the quasi-resonant switching power supply device shown in FIG. 1 when the load is none. As shown in FIG. 4, the switching element 5 performs the switching operation at a frequency as high as approximately 250 kHz when the load is none.
If an electronic instrument or the like connected to the Vout terminal is in a standby state, then an output load connected to an output terminal of the power supply becomes light. Therefore, the power to be supplied to the load is saved to be small in comparison with a usual operation state. However, at the time when the load is light, the switching frequency rises significantly as mentioned above. Therefore, a switching loss in the switching element 5 is increased, and in addition, there occur problems of damage to the switching element 5 owing to heat generation thereby, and of noise regulations in a frequency band concerned therewith. In this connection, a switching power supply has been proposed, which suppresses the rise of the frequency by defining an upper limit of the maximum switching frequency.
In Patent Publication 1, a switching power supply control circuit that improves power efficiency is disclosed. This switching power supply control circuit includes: signal generation means for generating a switching command signal for a switching element; oscillation means for oscillating a fixed cycle that defines an upper limit frequency at the time of turning on the switching element by the switching command signal; counting means for counting the number of times that the switching element is turned on so as to stop the switching command signal when the switching element is turned on a present N number of times in the case where the switching element is continuously turned on by the switching command signal. Then, in the case where the load is light, the switching element performs an intermittent operation, in which the switching element turns on continuously the number of N times, and is then paused.
Hence, in accordance with this switching power supply control circuit, the intermittent operation is performed a predetermined number of times by using a timer circuit as the oscillation means and a pulse counter circuit as the counting means, whereby the power efficiency can be improved. Moreover, the number N of continuous switching times is set at an appropriate number of times in response to a usage purpose of the switching power supply, whereby the switching power supply can be set into the optimum switching operation state with regard to suppression of a ripple and efficiency enhancement when the load is light, which are settled in a tradeoff relationship in a partial resonance power supply.    [Patent Publication 1] Japanese Patent Laid-Open Publication No. 2007-215316